This invention relates to a catalyst for the reduction of nitrogen oxides in lean exhaust gas of motor vehicle engines. The catalyst contains, on an inert structure reinforcing body, a first catalytic coating of aluminum oxide and/or cerium oxide with high surface area. The aluminum oxide and/or cerium oxide optionally may be stabilized with rare earth metals and/or silicon dioxide. This first catalytic coating layer acts as a carrier for the catalytically active noble metal components. A second catalytic coating of a zeolite is formed on the first coating layer.
The emission of the noxious substances carbon monoxide hydrocarbons and nitrogen oxides in internal combustion or Otto engines has been drastically reduced in recent years by means of controlled, three-way catalysts.
Three-way catalysts are capable of oxidizing carbon monoxide and hydrocarbons to the harmless compounds carbon dioxide and water, and at the same time, reducing nitrogen oxides to molecular nitrogen (see, e.g., German Patent No. 38 30 318, which is entirely incorporated herein by reference). The existence of a stoichiometric ratio of reducing and oxidizing components in the exhaust gas is a precondition for the satisfactory operation of three-way catalysts. This optimum ratio prevails when the "lambda number" or "air number" in the exhaust gas is equal to one. The air number lambda can be calculated, for example, from the various components in the exhaust gas by the Brettschneider formula (J. Brettschneider, Bosch Techn. Berichte 6 (1979) 177, which is entirely incorporated herein by reference).
Under real driving conditions, the air number for three-way catalysts must be constantly re-adjusted to the value 1 by a lambda control. For this purpose, the oxygen concentration in the exhaust gas is measured by a lambda probe, and the air-fuel ratio at the intake manifold of the engine is regulated so that the air number in the exhaust gas becomes equal to one.
However, this principle cannot be used for exhaust gas purification in diesel engines and so-called Otto lean engines. These engines always operate with a high oxygen excess which results in air numbers greater than one in the exhaust gas (lambda &gt;1).
"Diesel oxidation catalysts" as described, for example, in German Patent No. 39 40 758 (which is entirely incorporated herein by reference), have been developed for the purification of exhaust gases in diesel engines and lean engines. These catalysts have high conversion rates for the oxidation of carbon monoxide and hydrocarbons, but do not alter the nitrogen oxide content in the exhaust gas. A diminution in the nitrogen oxide content by reduction is difficult with these catalysts because of the high proportion of oxygen in the exhaust gas.
The reaction of nitrogen oxides with ammonia is described in German Patent Publication No. 36 35 284 (which is entirely incorporated herein by reference). Such a process is very difficult to employ in mobile sources of emission, because it requires an additional container for NH.sub.3 and a complicated dosing device. Moreover, it would appear inadvisable for safety reasons to carry ammonia in vehicles.
Further, secondary emissions of unreacted ammonia (ammonia leakage) are liable to occur in this system due to higher than stoichiometric ammonia dosing. Furthermore, the temperatures required for high conversion rates of the nitrogen oxides are above 400.degree. C. in this process.
Japanese Specification JP 1127044 describes a catalyst coating which is capable of oxidizing the carbon monoxide and hydrocarbons contained in the exhaust gas as well as substantially reducing the nitrogen oxides to nitrogen, in spite of the oxidizing exhaust gas conditions. This document is entirely incorporated herein by reference. The coating is a double coating having a first catalytic layer for the catalysis of oxidation reactions and a second layer of zeolite applied to the first layer. In a further step of the coating process, copper is applied as an active component to the second layer.
The first layer may consist of aluminum oxide and an oxide of a rare earth metal, e.g., cerium oxide, and is impregnated with one or more metals from the group of platinum, palladium and rhodium. After impregnation of the first layer with the noble metals, the second layer, consisting of zeolite and silica gel, is applied. Only then is the catalyst charged with copper by immersion of the entire preliminary catalyst in an aqueous copper acetate solution for 24 hours.
Although this catalyst known in the art converts the noxious substance NO.sub.x at exhaust gas temperatures in the range of 500.degree. to 600.degree. C., these temperatures are substantially above the temperatures of 225.degree. to 400.degree. C. typically occurring in exhaust gas, in particular from diesel engines. Data for the relevant exhaust gas temperature range for diesel engines in the range of 225.degree. to 400.degree. C. is not given in the Japanese Specification JP 1127044. Moreover, the subsequent impregnation of the whole carrier does not ensure that copper is deposited only on the zeolite and not on the aluminum oxide, as can be seen from the quantity of copper given in the Japanese Specification, which is 20 grams per liter of carrier volume. This is an extremely large quantity and exceeds the maximum exchange capacity of the zeolite. Further, homogeneous distribution of the copper over the monolith, in particular in the channels of the honeycomb, does not appear to be ensured by this step of the process.
Impregnation of the zeolite with copper is also very expensive and time consuming (24 hours according to the Japanese Specification).